Binding of granulocyte colony-stimulating factor (G-CSF) to its cognate receptor results in activation of multiple signal transduction pathways that are critical for the regulation of granulopoiesis. How G-CSF-activated signaling events are downmodulated is largely unknown. Mutations in the G-CSF receptor gene leading to its C-terminal truncation have been reported in patients with acute myeloid leukemia (AML), providing the first clinical evidence for the involvement of abnormal cytokine receptors in human leukemia. Expression of the truncated receptors in hematopoietic cells results in enhanced cell proliferation and survival but impaired myeloid differentiation in response to G-CSF. The truncated receptors also mediate dramatically augmented and/or prolonged activation of Stats and Akt, which play crucial roles in regulating cell proliferation and survival. Interestingly, substitution of the three C-terminal tyrosine residues with phenylalanine leads to prolonged but not enhanced activation of Stats and Akt whereas deficiency of tyrosine phosphatase SHP-1 is associated with enhanced but not prolonged Stat activation by G-CSF. We hypothesize that the magnitude and duration of Stat and Akt activation by G-CSF are downregulated by distinct negative regulatory mechanisms and that the C-terminal region of the G-CSF receptor may contain critical subdomains implicated in interactions with the negative regulatory mechanisms. Two specific aims are proposed to test this hypothesis: (1) to define the C-terminal subdomain required for the downregulation of the magnitude of Stat activation and examine the mechanism by which SHP-1 exerts its negative effect on Stat activation; and (2) to identify the C-terminal tyrosine residues involved in the negative regulation of the duration of Stat and Akt activation by G-CSF and investigate the roles of negative regulatory molecules such as SHP-2 and SHIP in G-CSF receptor signaling. The proposed research will improve our understanding of the regulatory network of G-CSF signaling and the molecular mechanisms by which C- terminal truncation of the G-CSF receptor contribute to leukemogenesis.